Sun
pillars change as the sun descends. At first the lower pillar
is strongest but as the sun descends the upper pillar grows taller
and brighter. The subhorizon
components are visible in nearby diamond
dust, from mountainsides
or from aircraft.
Simulations by HaloSim;
plate crystals of various tilts**,
10 million rays traced per pillar.

Upper pillars are short and faint when the sun is a few degrees high.
The sun needs to reflect off the underside of plate crystals
to make an upper pillar and when its rays are slanting downwards a
few degrees, only strongly tilted crystals have their lower faces
lit. In contrast, sunlight easily shines on upper faces and so the
lower pillar is strong. Most of the lower pillar is of course
beneath the horizon. The subsun - at the
same distance below the horizon as the sun is above it - is especially
bright.

As the sun sinks, the upper pillar lengthens and brightens. At sunset
both pillars are equal.

After the sun has set it still illuminates high clouds in the west.
These almost horizontal or even slightly upward going (from atmospheric
refraction) rays illuminate the lower crystal faces. They
reflect downwards to form a strong upper pillar. Watch a pillar
strengthen after sunset (the reduced glare and skylight also helps
make the pillar more apparent) and creep slowly northward following
the sun hidden below the horizon.

**

Large crystal tilts produce tall pillars but the distribution
of tilts also affects pillar appearance. Crystals in the simulation
had a uniform distribution of tilts up to ±3° from horizontal
then tailing off so that a few crystals had tilts of ±6°.
Other tilt distributions produce pillars of different appearance.
Try it in HaloSim.